Microwave frequency transmissions require the use of filters in transmission and in reception to select the frequency band in which the signal is transmitted. At microwave frequencies, it is possible to use guide-mode filters which make it possible to obtain low losses and a great selectivity.
In certain applications, it is advantageous to be able to tune the filter within a frequency band in order to be able to configure the hardware or the device at any moment and until operation according to the frequency of the signal to be transmitted.
There are a number of ways of producing guide-mode filters. Some use transverse partitions forming irises of inductive or capacitive type, others use longitudinal partitions (septums). The narrowest filters may have a relative bandwidth of a fraction of a percent of the center frequency.
The U.S. Pat. No. 5,808,528 [4] describes a bandpass filter which comprises a waveguide having a number of conductive walls and a moving wall defining the “large” dimension “a” of the waveguide. In this patent, use is made of the discontinuities created by a septum T with inductive obstacles in the vicinity of the axis of symmetry of the guide to define the cavities and the couplings of the filter (FIG. 1a).
The equivalent diagram of a guide cavity represented in FIG. 1b in accordance with the prior art [2] page 697 in which L is a line section of admittance Yo and jB is a line-end admittance.
The devices according to the prior art comprise cavities with inductive admittances at the end (produced by means of irises or of septums), for which the values of the equivalent inductive admittances (jB) at the ends of the cavities:B/Yo˜−(λg)/a*cot2(πd′/(2a))(in which “a” and “d” are defined in FIG. 1c) depend directly on the dimension of the large side of the guide “a” and vary considerably when “a” varies when the small side “b” of the guide is displaced parallel to itself to adjust “a”. FIG. 1c represents an example of an inductive iris according to the prior art.